1. Field of the Invention
The present invention relates to a discharge light-emitting device that emits light due to discharge at a space between electrodes charged with any discharge gas such as xenon, and to a contact image sensor utilizing the discharge light-emitting device as a light source.
2. Description of the Related Art
Hitherto, a discharge light-emitting device utilizing spontaneous discharge of gas, with which a space between electrodes is charged, has been known as a light source employed in a contact image sensor for reading graphic form or the like. Generally in the contact image sensor, any original held between a platen and a glass plate is irradiated with a discharge light-emitting device, and reflected light thereof passing through a rod lens array is transformed into electrical signals, thus the original comes to be read.
This known discharge light-emitting device is constructed such that any discharge gas such as xenon is charged into a space between two substrates opposed to each other. When ac current of about 1 to 2 KV is applied to the space between electrodes disposed on each substrate, discharge gas is ionized and discharges ultra-violet rays by which a fluorescent substance is energized and emits light.
FIG. 6 is a cross-sectional view showing a conventional discharge light-emitting device disclosed in the Japanese Patent Publication (unexamined) No. 2002-134064. Referring to this drawing, a discharge light-emitting device 1 includes a first substrate 2 composed of a glass plate. A first electrode 3 is formed on this first substrate 2. A dielectric layer 4 is formed on the first substrate 2 so as to cover the first electrode 3. A second substrate 6 includes a recess formed by a flat facing part 6a, an inclined part 6b and a leg part 6c. The second substrate 6 is mounted on the first substrate 2, thus forming a discharge space 9 with the recess.
A second electrode 7 is formed on the external surface opposite to the discharge space 9 formed on the second substrate 6. In the discharge space 9, a fluorescent layer 5 is composed of a lower fluorescent substance layer formed on the dielectric layer 4 and an upper fluorescent substance layer formed on the second substrate 6. A sealing layer 8 forms the discharge space 9 through bonding the first substrate 2 and the second substrate 6. The discharge space 9 is charged with any discharge gas such as xenon. The inclined part 6b is formed at a portion joining to the facing part 6a of the second substrate 6 and to the first substrate 2 so as to make about 45 degrees with respect to the facing part 6a of the second substrate 6. The fluorescent layer 5 is not formed at the discharge space 9 portion on this inclined part 6b. Accordingly, discharge light generated in the discharge space 9 outgoes through the inclined part 6b to outside.
The first substrate 2 is rectangular in external shape, and the rectangular first electrode 3 is formed in the longitudinal direction thereof. One end in the longitudinal direction of this first electrode 3 is connected to a high voltage power source located outside. The dielectric layer 4 is rectangular in external shape covering the first electrode 3. The fluorescent layer 5 is rectangular in shape and is formed on the dielectric layer 4.
FIG. 7 is a sectional view showing a construction of a contact image sensor in which the discharge light-emitting device shown in FIG. 6 is used as a light source. In the drawing, the discharge light-emitting device 1 acts as a light source of a contact image sensor 10. Referring to FIGS. 6 and 7, a housing 12 includes a mounting part 18 on which the discharge light-emitting device 1 is mounted, and supports horizontally thereon the first substrate 2 of the discharge light-emitting device 1. A glass plate 15 is also supported on the housing, and the second substrate 6 of the discharge light-emitting device 1 is disposed in close contact with the lower side of the glass plate 15. The inclined part 6b of the discharge light-emitting device 1 is disposed at a position near the irradiation point of an original 16 carried by a platen roller. On the supposition that inclination of the inclined part 6b is 45 degrees, light outgoing from this inclined part 6b comes to be the irradiation point. As indicated by the arrows in the drawing, the light reflected from the original 16 is converged through into the rod lens array 14, and contents of the original 16 are read through photoelectric transfer by a sensor 13 mounted on the circuit board 11.
In general, brightness at the original surface (original surface illuminance) is in proportion to brightness of light source and in inverse proportion to the square of distance between light source and original surface. Therefore, if it is possible to shorten the distance between the light-emitting surface of the light source and the original surface, the original surface illuminance can be increased even in the case of using the light source of the same brightness. Further, when reducing unnecessary space between the light source and original, it becomes possible to restrain disturbance due to reflection and disperse from the portions other than the original to be read, thereby enabling to read the original with high accuracy.
However, in the construction in which the discharge light-emitting device 1 is built in only on one side of the rod lens array 14 as shown in FIG. 7, a shadow may be projected due to corrugation or level difference in the original 16. This shadow may make the read contents of the original 16 indefinite or unclear, and moreover brightness of the discharge light-emitting device 1 may be insufficient. To cope with this problem, it may be an idea that the discharge light-emitting device 1 is built in on each of the two sides of the rod lens array 14. It is certain that, as a result of such arrangement, the indefiniteness due to the shadow of corrugation is overcome, and brightness is improved. But a problem exists in that increase in cost is unavoidable.